Sensitive light detectors are needed in various fields, such as quantum communication, space communication, astronomy, motion detection, molecule sequencing and others. The ultimate light sensitivity is reached when detecting a single quantum of light, the photon. Achieving reliable single photon detection at a high rate requires a high detection efficiency, a fast response, as well as negligible dark counts. Superconducting nanowire single photon detectors (SNSPD) currently are one of the devices used to provide such performance. Such SNSPDs are typically constructed of a thin (≈5 nm) and narrow (≈100 nm) superconducting nanowire, generally formed on a substrate such as silicon, sapphire, magnesium oxide, glass or the like, by means of conventional microelectronic fabrication techniques. The length is typically hundreds of microns, and the nanowire is patterned in a compact meander geometry to create a square or circular pixel with high area filling factor. The nanowire is cooled below its superconducting critical temperature and biased with a DC current that is close to but less than the superconducting critical current of the wire. The detection mechanism utilizes a fast avalanche process, in which an absorbed photon incident on the nanowire breaks hundreds of Cooper pairs and reduces the local critical current below that of the bias current. This results in the formation of a localized non-superconducting region, or hotspot, with finite electrical resistance. This resistance is generally larger than the load resistance (typically of 50 ohm), and hence most of the bias current is shunted to the load resistor. This produces a measurable voltage pulse that is approximately equal to the bias current multiplied by the load resistance (typically 50 ohms). With most of the bias current now flowing through the load resistor, the non-superconducting region cools and returns to the superconducting state. The time for the current to return to the nanowire is typically set by the inductive time constant of the nanowire, equal to the kinetic inductance of the nanowire divided by the impedance of the load resistor. Proper self-resetting of the device requires that this inductive time constant be slower than the intrinsic cooling time of the nanowire hotspot. However, the reset time should be kept to the minimum imposed by this limit; the detection time is inversely proportional to the detection rate. Hence, detector length should be kept to minimum to decrease the inductive time. Typical reset time scales of SNSPDs can be of the order of tens of picoseconds, requiring detection rates of well over the GHz. range. Current SNSPDs can thus provide fast response, as well as negligible dark counts, but suffer from detection efficiencies well below 100%.
The detection efficiency η, also referred to as the quantum efficiency, is defined as the percentage of photons detected by the detector, out of those received. The detection efficiency can be calculated as η=ηC×ηA×ηP, where: the coupling efficiency ηC is the percentage of photons impinging on the detector element itself out of those received at the optical input, the absorption efficiency ηA is the percentage of photons absorbed in the detector out of those impinging on it, and the pulse efficiency ηP is the percentage of photons creating a pulse out of those absorbed.
There are several prior art means of inputting light to be detected by the detector. One type of prior art detector systems input the light by means of free space coupling between the detector and the source, using complex lens systems. Other systems input light from a fiber. In order to ensure good coupling to the device (high ηC) one method used in the prior art is to align the end of the input fiber with the device using precise piezoelectric motors, whose positioning is controlled by a feedback mechanism based on the output level resulting from a probe illumination beam used to align the fiber end. Either of these systems is complex, costly, and slow to operate. Alternatively, the fiber end is roughly fixed relative to the device, and the light is focused onto the SNSPD device in the cryostat. These methods may result is light losses of the order of 50% or more.
A photon impinging upon the detector has a limited chance of being absorbed by the nanowire structure of the detector, because of the partially transparent nature of the thin layer of detector superconductor. In order to increase the value of the pulse absorption efficiency, ηA, it has been proposed in a number of prior art references to use an optical cavity, with the detector element itself acting as one mirror and a second metallic mirror disposed opposite it, or with a simple reflective element added opposite the detector element. Thus, for instance, in the article entitled “An Ultra-low Dark-count and Jitter, Superconducting Single Photon Detector for Emission Timing Analysis of Integrated Circuits” by P. LeCoupanec et al., published in Microelectronics Reliability, Vol. 43, pp. 1621-1626 (2003), an aluminum mirror is deposited onto the SSPD active area, to retro-reflect photons transmitted through the NbN layer, to increase its apparent thickness. In U.S. Pat. No. 6,812,464 to R. Sobolewski et al, for “Superconducting Single Photon Detector”, a concave mirrored surface is used to reflect and focus any photons which have passed through the SNSPD element without being absorbed, back onto the SNSPD element. In the article entitled “Nanowire Single Photon Detector with an Integrated Optical Cavity and Antireflection Coating” by K. M. Rosfjord et al, published in Optics Express, Vol. 14, No. 2, pp. 527-534 (2006), there is described an optical cavity fabricated on top of the detector structure, using a titanium/gold mirror on the detector surface remote from the optical input surface.
In general, these cavities have resulted in only a modest increase in the detection efficiency presumably because the detector element itself has a substantially transparent section. Moreover, the light is not generally confined in the directions perpendicular to the beam propagation direction, which result in light loss to the sides. As such the cavities produced have low finesse; the number of traverses of a photon within the cavity before it escapes without being absorbed by the NbN meander element is limited.
Furthermore, the above described prior art devices still require a sensitive and time consuming alignment procedure to ensure that the light input fall entirely on the SNSPD element.
There therefore exists a need for novel low light level detection devices, and especially SNSPD devices which overcome at least some of the disadvantages of presently available detectors.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.